Friday, September 27, 2013

Astro-Tech 8" Imaging Newtonian, Part I

I've been struggling with the very strong light pollution in Singapore, which forces me to image in narrowband.  I have resorted to 20-minute long sub-exposures using our Astro-Tech AT90EDT, which has been overall a good experience thanks to the Mach1, but at f/6.7 the depth of the images leaves something to be desired.

Obviously the option is to go faster (in terms of focal ratio).  However the Takahashi FSQ-106ED with reducer (f/3.6) and the Borg 125SD with Super Reducer (also f/3.6) are well beyond my wallet's capacity.  I also thought about the Starizona Hyperstar system which is really fast at f/2.0, but our C9.25 is not Hyperstar-capable, so I'd have to find a compatible SCT, on top of buying the adapter itself which is rather pricey as well.

Another option was the Takahashi Epsilon, and the new Epsilon 130D (to be introduced this November 2013) is in the realm of affordability (i.e. half the cost of the Epsilon 180ED). But the problem with the Epsilon 130D is that it is fairly short at 430mm - that's too wide-field for me!

I had been thinking about the Boren-Simon Powernewt, but it also is a pricey solution.  However an Astro-Systeme Austria 2" Keller reducer suddenly popped up on Astromart a few weeks ago.  It was definitely not cheap - in fact it is the most expensive astronomical item I've purchased in dollars per kilogram of weight.  These reducers come up used very rarely and are quickly snapped up, so I moved fast to get it.

Having obtained the Keller reducer, which has a 0.73X reduction ratio and also corrects for coma, it was time to find a Newtonian.  I decided on an 8" f/4 because I didn't feel too comfortable with the size and weight of the 10" - even though the Mach1 in theory should not break a sweat with the 10" model.  Luckily one of my Cloudy Nights forum buddies had an Astro-Tech AT8IN 8" f/4 Imaging Newtonian for sale. I got it at a good price (a stroke of good luck, as it's been back-ordered at Astronomics for many months now). But as usual shipping was expensive.

The QHY8 camera I've been using for the past few years had a nosepiece that provides exactly 55mm of back-focus.  This is for the Baader MPCC coma corrector (and all the generic refractor flatteners and reducers out there).  I'd been using the QHY8 with an Orion non-reducing refractor flattener, and this worked well with round stars to the corners.

I had already received the Keller reducer prior to the arrival of the AT8IN, but I had not factored in the 65mm required back focus of this reducer. Mismatching the back focus would completely undo the coma-correcting features of this corrector that I'd paid so much for. I don't have any extension tubes and  I didn't want to wait a long time for them.

Luckily I had this spare 2" to T-thread female adapter lying around from my misbegotten on-axis guider experiments.  The female T-thread allows the Keller reducer to screw into it, and the 2" eyepiece holder allows it to grip the QHY8 camera's nosepiece.  I used the pipe clamp as a crude parfocalizing ring, so that every time I remove the 2" to T-thread adapter, I can put it back and still get the 65mm (more or less) back focus without whipping out the digital caliper every time.

And voila:

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